1. Field of the Invention
The invention relates to an optical modulator, and more particularly, to an optical modulator for compensating for wavelength dispersion of optical fiber.
2. Description of Related Art
In the fields of optical communication or optical measurement, an optical signal wave modulated in an optical modulator is transmitted in an optical fiber. In the optical fiber, since the propagation velocity of light or the length of the propagation path is different depending on the wavelength, wavelength dispersion occurs and the waveform of the optical signal is distorted. Particularly, waveform distortion is very obvious in the SSMF which is a standard single mode fiber. As a result, the technology for compensating for wavelength dispersion of the optical fiber is indispensable in high-speed communications over 40 Gbps, high-speed wavelength multiplexing transmission systems, and the like.
As a method of compensating for dispersion, there are known methods of arranging dispersion compensation fiber immediately before a receiver of an optical signal, methods of using Fiber Bragg Grating (FBG) or optical etalon devices as disclosed in Japanese Unexamined Patent Application Publication No. 2004-12714 or using a digital signal processing circuit as disclosed in Japanese Unexamined Patent Application Publication No. 2010-226254 or Robert I. Kelley, et al., “Electronic Dispersion Compensation by Signal Predistortion Using Digital Processing and a Dual-Drive Mach-Zehnder Modulator,” IEEE Photonics Technology letters, Vol. 17, No. 3, pp 714-'716, 2005. The digital signal processing circuit generates impulse responses for compensation in the digital signal processor depending on a change of a real part or an imaginary part related to the wavelength dispersion.
In the dispersion compensating fiber, the compensation precision is limited by the minimum unit of the amount of compensation. In the wavelength dispersion compensation of the light waves of wavelength division multiplexing (WDM) system, the optical device such as FBGs are separately needed as the wavelength dispersion compensators in order to compensate after dividing WDM light waves. In addition, the optical device of FBG and the like has not only the limitation to operation band of the wavelength but also the large optical loss. Furthermore, in the digital signal processing circuit, the high-speed processing over 40 Gbps is the difficult technical problem.